Capacitive membrane ultrasonic transducers have a metal coated membrane such as silicon or silicon nitride supported above a substrate by an insulating layer such as silicon oxide, silicon nitride or other insulating material. The substrate may be a highly doped semiconductor material such as silicon or may be undoped silicon with a metal layer. The thin metal covering the membrane and the highly doped substrate or metal layer form the two electrodes of a capacitor. Generally the substrate, support and membrane form a cell which may be evacuated. Generally the transducers comprise a plurality of cells of the same or different sizes and shapes. In operation, the cells may be arranged in arrays with the electrical excitation generating beam patterns. Typically transducer cells have sizes ranging between 5 μm and 1000 μm in diameter.
The fabrication and operation of capacitive membrane transducers is described in many publications and patents. For example U.S. Pat. Nos. 5,619,476, 5,870,351 and 5,894,452, incorporated herein by reference, describe fabrication using surface machining technologies. Pending Application Ser. No. 60/683,057 filed Aug. 7, 2003, incorporated herein by reference, describes fabrication by using wafer bonding techniques. Such transducers are herein referred to a capacitive membrane ultrasonic transducers (cMUTS).
FIG. 1 shows a typical cMUT cell. The active part of a cMUT is the membrane 11 with metal electrode 12 supported above a fixed substrate 13. A DC bias voltage 14 is applied between the membrane and the bottom electrodes to create electrostatic attraction, pulling the membrane toward the substrate. An AC voltage 16 is applied to the biased membrane to generate harmonic membrane motion and ultrasonic waves.
In operation, it has always been assumed that the substrate is fixed (not movable) and an applied alternating electric field forces the membrane into motion, thus generating ultrasonic waves in the medium, whether air, water, or a similar medium. However, this assumption does not hold in the entire frequency range. Both experiments and simulations show that there is always some energy coupled into the substrate generating longitudinal waves which reflect from the bottom of the substrate, and are picked up by the capacitor. The waves that are reflected from the bottom of the substrate are out of phase with the excitation signal except at frequencies (Eq. 1) where the thickness of the substrate is an integer multiple of half of the wavelength. That is, the energy coupled into the substrate is not a concern except at those frequencies where the substrate will behave like a resonant cavity and hinder proper device operation. These are the frequencies where the substrate thickness is a multiple of the half of the wavelength:
                              f          R                =                              n            ·            v                                2            ⁢            t                                              (        1        )            where n is an integer number, v is the longitudinal velocity of sound in the substrate, and t is the thickness of the substrate. For example, if we assume that the substrate is silicon and the longitudinal wave velocity is 8000 m/s, and the thickness of the substrate is 500 μm, then these substrate ringing modes will occur at integer multiples of 8 MHz. If the operating frequency range of the cMUT transducer is below the first ringing mode (8 MHz in the above example), or between the ringing modes, then there is no concern for bulk wave generation (FIG. 2, case 1 and 2). However, if the operating frequency range contains one or more of these modes, then these modes will appear as notches in the frequency response (FIG. 2, case 3), and cause ringing in the time-domain response of the cMUT transducer, which is certainly undesirable. One way to eliminate the substrate ringing modes is to bond the silicon substrate to a backing as suggested in U.S. Pat. No. 6,862,254. In this method, the backing has to be impedance matched to silicon and attenuating. However, the attenuation coefficient cannot be too high because it deteriorates the impedance matching. Therefore, to achieve adequate suppression for the substrate ringing modes one has to have a thick backing layer. This method has been shown to work properly. However, carrying a thick backing layer on the back of a normally thin cMUT transducer is not practical for many applications where space is limited. The bonding of the cMUT transducer to the backing is achieved using an epoxy layer as glue. Proper suppression of the substrate modes requires very thin epoxy layer at the bonding interface, and a hermetic bond. Although these requirements may be relatively easy to achieve at low frequencies, the requirements become tighter as the frequency increases.